the best way ist to really post the image, which means using the dedicated image field below the content entry field. You have to have the image on your local hard drive and it has to be less than 100 KB in size. You have to use the button right of the image entry field to navigate to the file on your local hard drive. You can only have one image per message this way, but it not only shows up in the index but you also get a preview when you put your mouse pointer over an image number (works at least with the Firefox browser).
The second best way is to put an image link in your message. You can put in as many links as you want. They now also show up in your image index but you don't get previews (this is a bit of a copyright problem. Since the image is on an image hosting server, I would have to pull a copy to provide a preview. But if Google does it, maybe I get away with it too?). The way to to this is to enclose the image in [img] tags. [img]http://www.server.tld/image.jpg[/img] should do it.